For example, in a Loran C receiver, electric waves of a Decca navigation system (hereinafter referred to as the Decca electric wave) may sometimes lie in the vicinity of its receiving frequency band. For rejecting this Decca electric wave, a narrow-band rejection filter, which is what is called a notch filter, is employed. A vessel loaded with the Loran C receiver sails passing through areas of Decca electric waves of different frequencies, so that when it enters an area in which the frequency of the Decca electric wave jamming reception differs from that in the previous area, it is necessary to adjust the center frequency of the abovesaid narrow-band rejection filter to accurately set it to the jamming wave. Since the rejection filter is to reject the jamming wave in the vicinity of the receiving band or in the receiving band, its rejection band designed to be narrow; accordingly, the operation of adjusting its center frequency to the frequency of the jamming wave is very difficult and much time and skill are required for the adjustment; in addition, even if this adjustment contains a slight deviation, the presence of this rejection filter rather rejects a portion of the signal component desired to receive and the jamming wave rejecting effect extremely lessens, making it impossible to achieve the desired object. The jamming wave to the Loran C receiver, the Decca electric wave in general, may sometimes exist not only at one frequency but also at a plurality of frequencies and it was very troublesome to adjust the center frequency of the narrow-band rejection filter for each of them. In ordinary receivers as well as in the Loran C receiver, the adjustment for rejecting the jamming wave by a narrow-band rejection filter was very cumbersome and demanded time and skill.
An object of this invention is to provide a jamming wave rejecting device which is designed so that when the frequency of a narrow-band rejection filter is brought to the neighborhood of the jamming wave to be rejected, the center frequency of the narrow-band rejection filter automatically and accurately coincides with the frequency of the jamming wave, thus enabling rejection of the jamming wave easily and accurately.